'exec' function is disabled in your PHP configuration. You will not be able to generate PDF using PDF (Ghostscript, level 1.2) or PDF (Ghostscript, level 1.4) output methods, as these output methods require GNU Ghostscript or AFPL Ghoscript executables to be run on your server.

'exec' function is disabled (please note that it have nothing to do with the PHP safe mode; particular functions can be disabled even when safe mode is OFF). Enable 'exec' function in your php.ini (refer your PHP manual or for exact instructions)
Try using output methods not requiring running executables on your server.